In Spring Boot, configuration files can use .yml or .yaml as the file suffix, they are equivalent without any difference. In fact, they are all files in YAML (YAML Ain't Markup Language) format, but the file suffix names are different.
YAML is a human-readable data serialization format, often used for writing configuration files. Spring Boot supports the use of configuration files in YAML format instead of traditional .properties or .xml files to provide a more concise and readable configuration method.
You can choose to use .yml or .yaml file extensions as Spring Boot configuration files, depending on your personal or team preference. Both work fine and have the same syntax and functionality.